Digital communication systems, such as wireless communication systems, and digital storage systems, such as hard disk drive systems, transfer information in the presence of noise. Improving the accuracy of information transfer in such systems may entail the use of complex error recovery techniques, such as the use of elaborate error correction codes.
Digital magnetic recording stores digital data by modulating a magnetic flux pattern in a magnetic medium. During the storing process, an electric current in a write head is modulated based on the digital data to be written. The head is positioned over magnetic material in the shape of a circular disk which rotates rapidly. The electric current in the write head, in turn, modulates the magnetic flux pattern in the medium. The medium used is such that the flux pattern is retained in the medium after the electric current is turned off in the write head, thus providing data storage.
Data is usually written in the medium in concentric circles called tracks, which are further divided into user or read data sectors and servo sectors embedded between the read data sectors. The servo sectors contain data and supporting bit patterns required for control and synchronization. The control and synchronization information is used to position the magnetic recording head, so that the information stored in the read data sectors is retrieved properly. Being able to accurately read data is important to the operation and recovery of information in digital storage systems. To improve the accuracy of reading data, data is written to a medium using an error correcting technique, such as, an interleaved parity technique, which interleaves parity bits throughout the data to be stored.
During a process to read the stored data, a read head, for example, is positioned over the medium following the tracks. The magnetic flux patterns stored in the medium induce a varying current in the read head. This varying current is then processed to recover the written data, including the interleaved parity bits. Both the actions of writing data and reading data are susceptible to noise from various sources, including near-DC noise, which is noise of a relatively low frequency. To accurately retrieve the data, the process of interpreting the signals from the read head can use, for example, filtering, amplification, timing acquisition, and error correction techniques.
While perpendicularly recorded magnetic media allows for greater recording densities and improved data transfer performance, the challenge to accurately store and read data becomes more difficult. Even though perpendicularly recorded media poses a different set of problems than longitudinally recorded media, both storage technologies require efficient error correction techniques to minimize the effects of noise and detrimental media and read and write channel characteristics.